From the dissertation "Controlled Tissue-Expansion in Reconstructive Surgery" (Julian H. A. van Rappard, Thesis Groningen, The Netherlands, 1988) a tissue expander is known that is expanded by the gradual filling with a liquid. The tissue expander has an impermeable, stretch-resistant skin and a self-sealing valve for the filling with liquid using a hollow needle and a syringe. The tissue expander is implanted under the tissue to be expanded, with the valve arranged so that it is accessible by the needle from the outside. For the actual expansion of the tissue, the tissue expander is gradually filled with liquid. The needle is inserted into the valve through the tissue and the liquid is injected into the tissue expander with the syringe. When fully filled with liquid, the tissue expander obtains a shape determined by the form of its skin. The forming of the skin is adaptable to various applications in this way. It is an advantageous feature of the known tissue expander that a precisely controlled expansion of the tissue is possible. A major disadvantage, though, is the occurrence of high peak pressures after each filling of liquid into the tissue expander. This concerns especially the regions of tissue to be expanded located directly next to the tissue expander. These regions are compressed so much that damage of the tissue occurrs. When reducing the amount of liquid that is injected into the tissue expander in each step, problems result from the frequent piercing of the tissue in the region of the valve. Furthermore, the valve may develop a leak, which renders the tissue expander useless. There is no danger for the tissue surrounding the tissue expander as a result of a leaky valve, as long as a physiologically safe, sterile liquid for the filling of the tissue expander.
A self-inflating tissue expander of the type described above is known from the article "A Self-Inflating Tissue Expander" (E. D. Austad et al., Plastic and Reconstructive Surgery, Vol. 70 No. 5, pages 588 ff). This tissue expander consists of a silicone membrane filled with a sodium chloride solution. The molarity of the sodium chloride solution is greater than the physiological molarity of approximately 0.3. The osmotic driving force, which drives body liquid from the tissue surrounding the tissue expander through the semipermeable silicone membrane into the tissue expander, is based on this. The inflation of the tissue expander, and therefore also of the tissue surrounding the tissue expander, occurrs without the need of any manipulations from the outside. Furthermore, the tissue surrounding the tissue expander has an exceptional, undamaged quality after the expansion. The reason for this is that the self-inflating tissue expander does not create pressure peaks on the one hand, and that the intake of body fluid into the expander stimulates the metabolism of the surrounding tissue on the other hand. A disadvantage is the small amount of volume expansion of the tissue expander, at least as long as the molarity of the sodium chloride solution initially does not exceed a physiologically acceptable value by far. Another disadvantage is that the properties of the silicone membrane change with the expansion. Especially the pore size of the silicone membrane steadily increases. In this way an increasing amount of sodium chloride ions can pass through the silicone membrane. This leads to a decrease in the osmotic driving force, though, which is not coupled with a gain in the volume expansion of the tissue expander. A further disadvantage of the known tissue expander is the lack of a possibility to influence the direction in which the expansion of the tissue takes place. The form of the silicone membrane has only a minor influence on the shape of the tissue expander after its inflation. It is furthermore established that the inflation of the silicone membrane itself uses up a considerable amount of the osmotic driving force of the tissue expander. This is compounded by the fact that the stretching of the silicone membrane needs more strength as the volume of the tissue expander increases, while the osmotic driving force decreases at the same time. Only a strongly decreasing resulting driving force is then left for the expansion of the tissue surrounding the tissue expander, and the ratio of the initial size of the tissue expander to the attainable final size is further reduced beyond the calculated value.
From the U.S. Pat. No. 4,237,893 a device to widen the cervix is known. The device has a rod-shaped outer form and an at least three layered interior structure. An intermediate layer is made from a hydrophilic polymer material, i. e. a hydrogel. The device is introduced into the cervix and there expands by taking up body fluid from the uterus. By this the cervix is widened in its cross section. When the desired opening of the cervix has taken place after some hours the device is removed and a surgery can be performed through the cervix. The known device serves to temporarily widen an existing orifice of the body, but neither is a new orifice created, nor is additional tissue created. Furthermore, the known device to widen the cervix is not meant to be implanted in the tissue, but to be introduced into an already existing, open body orifice.
The U.S. Pat. No. 3,867,329 describes a method for making a rod-shaped body from hydrogel, which is supposed to serve as a device to widen the cervix. At first a copolymerisation of different aqueous substances is carried out and the resulting copolymer is then further treated. The resulting hydrogels have a swelling coefficient of up to 25 after 5 days in distilled water. Information about a swelling coefficient in a physiological sodium chloride solution is not contained in the U.S. Patent.
From the U.S. Pat. No. 3,975,350 it is known to use a hydrogel made from a polyurethane polymer as an implantable carrier of drugs. The aspect of tissue expansion is not mentioned in the U.S. Patent.
It is known to make so called soft contact lenses from hydrogel. Under the generic term hydrogel polymer substances are understood, which expand in an aqueous environment by taking up water. The amount of expansion is very different, depending on the hydrogel. It is quantified by the swelling coefficient. A swelling coefficient of n means that the initial volume has increased n-fold by taking up water. A constituent of the swelling coefficient is the naming of the solution in which it was determined. It is immediately seen that due to the higher osmotic pressure the swelling coefficient in distilled water will always be larger than in e. g. a physiological sodium chloride solution. The hydrogel from which soft contact lenses are made has a swelling coefficient of less than 4 in a physiological sodium chloride solution. It also has an advantageously high form stability and tear resistance in the swollen state.